Labeyrie ([CITE], A&A, 118, 517) established the feasibility of snapshot
images with a multi-aperture interferometer having a densified
exit pupil. The numerous widely spaced mirrors in these
instruments, called hypertelescopes, do not alleviate the usual
difficulty of adjusting and phasing interferometers. A
simplification is however possible, in the form of the optical and
mechanical architecture called Carlina (Labeyrie et al. [CITE],
Proc. SPIE, 4838). It
is configured like a diluted version of the Arecibo
radio-telescope. Above the diluted primary mirror, made of fixed
co-spherical segments, a helium balloon carries a gondola
containing the focal optics and detector. We describe in more
detail the Carlina concept, including versions equipped with an
equatorial drive and a coudé train. The optical design with a
clam-shell corrector of spherical aberration is optimized with a
ray-tracing code. A two-element prototype of a sparse aperture,
multi-element, optical dish has been built using a steerable
balloon-suspended secondary optical structure. Following imaging
and tracking tests with a single mirror, which give encouraging
results, fringes have been obtained on Vega with a pair of closely
spaced mirrors. We developed adjustment techniques for
co-spherizing the mirrors within one or a few microns, using a
light source at the curvature center. The absence of delay lines
is a major simplification with respect to conventional
interferometers, paving the way towards using hundreds or
thousands of sub-apertures for producing direct images with rich
information content. These results demonstrate the short-term
feasibility of large Carlina hypertelescopes, with effective
aperture size possibly reaching 1500 m at suitable terrestrial
sites. Such interferometers will provide snapshot images of star
surfaces, and of exo-planets if equipped with an adaptive
coronagraph. Collecting areas comparable to those of ELTs appear
feasible at a lower cost, while providing a higher resolution and
similar limiting magnitude.

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